1. Field of the Invention
The present invention relates to shielding workers from a tube plug ejection, during hydrostatic pressure testing of shell-and-tube heat exchangers. It replaces the channel cover during testing to allow observance of tube plugs to check for leakage.
2. Description of the Prior Art
The tubes in a Shell-and-Tube heat exchanger can corrode over time, allowing for the undesired mixing of fluids. When a tube corrodes enough to fail, the tube is plugged with a tapered metal plug, commonly referred to as a tube plug. These tube plugs are inserted into both ends of the tube at the tubesheet, which eliminates the use of that tube, but allows continued operation of the heat exchanger without the mixing of fluids. Following tube plugging, a hydrotest, or hydrostatic pressure test, of the exchanger shell side is usually conducted with the channel cover removed so that leaks can be detected on the tube side face of the stationary tube sheet. To detect leaks, a worker usually observes the tube sheet face for fluid leakage around the plug. In industry, there have been instances in which a tube plug in the stationary tube sheet became dislodged while under shell side pressure, ejecting the plug at a high velocity towards the observer. Typically a worker will position themselves behind a sheet of plywood or wire mesh, and peer around the edge to observe the tube sheet. This practice is unsafe, as these materials do not provide adequate protection in case of plug ejection. Without a standard, and proven safe shield, workers must rig some temporary shielding, including but not limited to the above mentioned materials, which can prove to be time consuming and inadequately supported. And, since the heat exchangers are usually used outdoors in a large industrial facility, they are not always easy to access for rigged temporary shielding, as they may be located in stacks extending high off the ground, with limited accessibility. An idea for an innovative, proven shield, which could be attached to the end of these heat exchangers during testing, was needed.
U.S. Pat. No. 6,401,427 discloses a modular system for containing projectiles, which has a sheet of material including at least a polycarbonate layer held by a metal frame having a straight frame member corresponding to each straight edge of the sheet. Each frame member has a U-shaped shield channel covering and holding a straight edge of the sheet and an adjacent U-shaped clamp channel rigidly held against the shield channel. A flexible gasket separates each sheet edge from its respective shield channel; and each frame member is fastened to each adjacent frame member only by clamps extending between adjacent clamp channels.
This system is limited to rectangular shielding and cannot be attached to the end of a heat exchanger efficiently. It, as with other shield materials, would need to be rigged in place, with a temporary support structure that would need to be modified depending on the location and installation specifics of the heat exchanger undergoing testing.
Polycarbonate and polycarbonate laminates are fabricated and used as impact resistant transparent barriers. There has been limited testing, with most tests occurring in the ballistic range of velocities of 1000-3000 ft/s. Manufacturer's data on impact resistance certifies polycarbonate materials in terms of Underwriters Laboratories (UL) or National Institute of Justice (NIJ) ballistic levels. Ballistic masses are confined to bullets, or bullet sized projectiles. To determine if a polycarbonate shield will resist a larger mass at a lower velocity, testing must occur.
A successful heat exchanger pressure test shield should accomplish two goals; most importantly, it must stop the tube plug upon impact. In addition, it should stop the tube plug with minimal movement, as the observer will be looking through the shield, and a large deflection could cause the shield material itself to hit the observer.